This is the data supplement presented in Nguyen et al. 2021 (https://doi.org/10.5194/essd-2021-213) in which we present 20-year flask sample records of atmospheric CO2, δO2/N2 and APO from the stations Lutjewad (the Netherlands) and Mace Head (Ireland) and a 3-year record from Halley station (Antarctica), including details of the extensive calibration procedure and its stability over time. The results of our inter-comparison involving gas cylinders from various research laboratories worldwide also show that our calibration is of high quality and compatible with the internationally-recognised Scripps scale. The measurement records from Lutjewad and Mace Head show similar long-term trends during the period 2002-2018 of 2.31 ± 0.07 ppm yr-1 for CO2 and -21.2 ± 0.8 per meg yr-1 for δO2/N2 at Lutjewad, and 2.22 ± 0.04 ppm yr-1 for CO2 and -21.3 ± 0.9 per meg yr-1 for δO2/N2 at Mace Head. They also show similar δO2/N2 seasonal cycle with an amplitude of 54 ± 4 per meg at Lutjewad and 61 ± 5 per meg at Mace Head, ...